Device for generating high-efficiency intrinsic circular dichroism and preparation method thereof

Abstract

The invention discloses a device for generating high-efficiency intrinsic circular dichroism and a preparation method thereof.The device comprises a substrate and a metasurface attached to the substrate, the metasurface is composed of nano-structures arranged periodically, the geometrical shape of each nano-structure is a double-rectangle combination, and the number of the nano-structures is two. The double-rectangle combination comprises a small rectangle and a large rectangle which are communicated with each other, the area of the large rectangle is larger than that of the small rectangle, and the geometrical shape and the arrangement mode of the nanostructure are configured to generate different optical responses to left-hand circularly polarized light and right-hand circularly polarized light; the nanostructure is in-plane mirror symmetry broken, and the nanostructure is partially etched to break out-of-plane mirror symmetry of the nanostructure so as to obtain the intrinsic circular dichroism. According to the device, the intrinsic circular dichroism efficiency larger than 0.9 is achieved within the near-infrared band from 1400 nm to 1700 nm, the broadband working capacity and the strong field localization effect are achieved, the mature nanometer processing technology is adopted, the preparation technology is simplified, and cost is reduced.

Description

Technical Field

[0001] The present invention relates to an apparatus for generating high-efficiency intrinsic circular dichroism and a preparation method thereof, belonging to the fields of micro-nano optics, laser technology, and optical sensing technology. Background Art

[0002] Photons with spin angular momentum have intrinsic chirality, which is the basis for many phenomena, including nonlinear optics, quantum optics, topological photonics, and chiral optics. When a chiral object interacts with light, a unique optical response can be induced: circular dichroism, which generally refers to the difference in the optical responses of a chiral medium to left-handed circularly polarized light (LCP) and right-handed circularly polarized light (RCP). Circular dichroism can be divided into extrinsic circular dichroism and intrinsic (internal) circular dichroism according to its origin. Extrinsic circular dichroism originates from the difference in cross-polarized optical signals, while intrinsic circular dichroism originates from the difference in co-polarized optical signals. To achieve extrinsic circular dichroism, it is necessary to break the in-plane or out-of-plane mirror symmetry of the structure (or add incident light or an external field at a specific angle), while to achieve intrinsic circular dichroism, it is necessary to break both the in-plane and out-of-plane mirror symmetries of the structure simultaneously (with normal incidence of light). Although extrinsic circular dichroism has its unique value in certain specific scenarios, its limitations such as dependence on external conditions and poor stability restrict its practical application range. Compared with extrinsic circular dichroism, intrinsic circular dichroism signals have more advantages in terms of stability, information directness, quantitative ability, and dynamic process analysis, and thus have broad application values in the fields of chiral optics, nano-photonics, biomedicine, etc.

[0003] The methods for obtaining intrinsic circular dichroism mainly rely on the inherent chirality of the molecular structure or nanostructure of natural materials itself, including chemical synthesis, biological templates, crystal growth, etc. However, since the signals of natural materials' chiral light responses are very weak, it is difficult to achieve high-efficiency intrinsic circular dichroism by the above methods. On the other hand, recent studies have found that resonant metasurfaces based on bound states in the continuum (BIC) can be used to enhance circular dichroism. BIC is a bound state that can achieve perfect localization of the electromagnetic field when the system's eigenstate is embedded in the optical radiation continuum. In theory, the BIC state is completely decoupled from far-field radiation, but by converting the ideal BIC state into a quasi-BIC state with a high quality (Q) factor, the interaction between light and matter can be effectively enhanced, thereby achieving a significant enhancement of the circular dichroism signal. However, in the past, in order to enhance the intrinsic circular dichroism signal by means of the quasi-BIC state, methods such as tilting or stretching the microstructures were required to break both the in-plane and out-of-plane mirror symmetries of the structure simultaneously. Such an approach, on the one hand, increases the difficulty of fabricating the microstructures, resulting in too high a preparation cost of the device and being unfavorable for large-scale production; on the other hand, due to the lack of a corresponding simplified analysis model for guidance, the design and regulation of high-efficiency circular dichroism, especially near-perfect circular dichroism, are blind. Summary of the Invention

[0004] To solve the above problems, the present invention provides a device for generating high-efficiency intrinsic circular dichroism and a preparation method thereof to generate high-efficiency intrinsic circular dichroism.

[0005] In a first aspect, the present invention provides a device for generating high-efficiency intrinsic circular dichroism, including a substrate and a metasurface attached to the substrate. The metasurface is composed of periodically arranged nanostructures. The geometric shape of the nanostructures is a combination of two rectangles, and the combination of two rectangles includes two small rectangles and a large rectangle that are interconnected. The area of the large rectangle is larger than that of the small rectangle. The geometric shape and arrangement of the nanostructures are configured to produce different optical responses to left-handed circularly polarized light and right-handed circularly polarized light. The nanostructures have broken in-plane mirror symmetry, and partial etching of the nanostructures is used to break the out-of-plane mirror symmetry of the nanostructures to obtain intrinsic circular dichroism. Among them, under the condition of non-breaking symmetry, the side length of the air hole of the nanostructure is l, the height of the nanostructure is H, and the out-of-plane mirror asymmetry parameter is defined as β = h / 2H, where h represents the etching depth. Breaking the in-plane mirror symmetry of the nanostructures changes to a combination of two rectangles with unequal areas, a large rectangle and a small rectangle, after changing the length of δ, and the in-plane mirror asymmetry parameter is defined as α = δ / l, where δ represents the width of the small rectangle in the combination of two rectangles.

[0006] In an embodiment of the present invention, the substrate is a low-refractive-index material, silica.

[0007] In an embodiment of the present invention, the nanostructures of the metasurface are made of silicon material, with a height of 500 nm, a width of 480 nm, and an etching depth of 345 nm.

[0008] In an embodiment of the present invention, the arrangement period of the nanostructures is 905 nm.

[0009] In an embodiment of the present invention, the metasurface is configured to generate circular dichroism in the near-infrared band, and its working wavelength range is from 1400 nm to 1700 nm.

[0010] In an embodiment of the present invention, the in-plane mirror asymmetry parameter α of the nanostructures is 0.177, and the out-of-plane mirror asymmetry parameter β of the nanostructures is 0.155.

[0011] In a second aspect, the present invention provides a preparation method for a device for generating high-efficiency intrinsic circular dichroism, and the preparation method includes:

[0012] Step 1: Coating a photoresist on the surface of the substrate and transferring the pattern of the nanostructures to the photoresist by electron beam lithography technology;

[0013] Step 2: Use dry etching or wet etching technology to transfer the pattern on the photoresist mask to the substrate or functional material layer, forming nanostructures with the above geometric shapes and dimensions;

[0014] Step 3: Remove the residual photoresist, and clean and surface-treat the metasurface to improve its optical performance and environmental stability.

[0015] In an embodiment of the present invention, the specific steps of Step 2 include:

[0016] Dry etching: Use reactive ion etching or inductively coupled plasma etching technology to transfer the pattern on the photoresist mask to the substrate or functional material layer; control the depth and sidewall morphology of the nanostructures by adjusting the flow rate, power, and etching time of the etching gas to ensure that their geometric shapes and dimensions meet the design requirements;

[0017] Wet etching: Use a chemical etching solution to selectively etch the substrate or functional material layer, and achieve high-precision shaping of the nanostructures by controlling the concentration, temperature, and etching time of the etching solution;

[0018] The specific steps of Step 3 include:

[0019] Removing the residual photoresist: Use an organic solvent or oxygen plasma cleaning technology to thoroughly remove the photoresist residue and ensure the cleanliness of the nanostructure surface;

[0020] Surface passivation treatment: Deposit a passivation film on the surface of the nanostructures by atomic layer deposition or chemical vapor deposition technology to improve their environmental stability and corrosion resistance.

[0021] In an embodiment of the present invention, the preparation method further includes the following quality control steps:

[0022] Morphology characterization: Use a scanning electron microscope or an atomic force microscope to characterize the morphology of the nanostructures to ensure that their geometric parameters meet the design requirements;

[0023] Optical performance testing: Test the optical performance of the metasurface by a circular dichroism spectrometer or an ellipsometer to verify the intensity and wavelength range of its chiral optical response;

[0024] Process optimization and iteration: Optimize and adjust the preparation process according to the test results until the desired intrinsic circular dichroism efficiency and optical performance are achieved.

[0025] In a third aspect, the present invention provides a simplified analysis model structure for evaluating the chiral quasi-bound states in the continuum circular dichroism in the device for generating high-efficiency intrinsic circular dichroism described above. Based on the coupled mode theory and time-reversal symmetry and energy conservation, the expression of the reflection component / transmission component is written in the following form:

[0026]

[0027] Among them, A j 、B j 、C j 、D j represent fitting parameters, γ is the attenuation rate caused by radiation, ω0 is the resonance frequency, and ω is the frequency of the incident light;

[0028] Define CD = R LL +R RL -R RR -R LR , for the intrinsic CD, due to the cross-polarization component R RL =R LR , so it is written as:

[0029]

[0030] Among them, the subscripts of R ij (i = R,L, j = R,L) respectively represent right-handed circularly polarized light and left-handed circularly polarized light, j represents the incident light, i represents the outgoing light, m and n are the resonance coupling coefficients of the structure with incident left-handed circularly polarized light and right-handed circularly polarized light respectively, and r' represents the background scattering coefficient.

[0031] The beneficial effects of the present invention are as follows:

[0032] 1. By designing a chiral quasi-BIC metasurface that reduces the preparation difficulty, the present invention significantly enhances the selective response to LCP and RCP, realizes high-efficiency intrinsic circular dichroism, and solves the problems of high preparation difficulty and insufficient response intensity of traditional chiral materials.

[0033] 2. The present invention optimizes the nanostructure design of the metasurface to achieve efficient chiral optical response in the near-infrared band from 1400 nm to 1700 nm. At the same time, by utilizing the strong field localization effect of the BIC mode, the interaction between light and matter is significantly enhanced, providing a basis for high-sensitivity optical sensing and detection.

[0034] 3. The present invention adopts mature nanofabrication technologies such as electron beam lithography, dry / wet etching, etc., combined with the design of the BIC mode, avoiding the complex engineering tilt requirements in traditional methods, simplifying the preparation process, reducing costs, and improving production efficiency.

[0035] 4. The present invention selects stable materials such as SiO2 substrates and Si nanostructures, which are abundant in source, low in cost and easy to process. Combined with surface passivation treatment, the environmental stability and durability of the metasurface are improved. At the same time, by controlling the unit to adjust the wavelength and polarization state of the incident light, the versatility and tunability of the device are realized, meeting the requirements of different application scenarios.

[0036] 5. The device provided by the present invention can be widely applied to the fields of chiral molecule detection, circularly polarized light detectors, chiral displays, biomedical imaging, etc., providing an efficient, stable and easy-to-integrate solution for the development of chiral optical technologies, and having important scientific value and industrialization potential. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0038] Figure 1 It is a schematic structural diagram, energy band diagram and corresponding Q factor diagram of the device for generating high-efficiency intrinsic circular dichroism proposed by the present invention.

[0039] Figure 2 It is a flowchart of the preparation method of the device for generating high-efficiency intrinsic circular dichroism in Embodiment 3 of the present invention.

[0040] Figure 3 It is an evolution diagram of the chiral C point in k space for the metasurface with different parameters α and β of the device for generating high-efficiency intrinsic circular dichroism proposed by the present invention.

[0041] Figure 4 It is a schematic diagram of the significant influence of reactive helicity density (RHD) in generating intrinsic chiral radiation. Among them, Figure 4 (a) in is the RHD distribution diagram of the metasurface with parameters α = 0 and parameter β = 0; Figure 4 (b) in is the RHD distribution diagram of the metasurface with parameters α ≠ 0 and parameter β = 0; Figure 4 (c) in is the RHD distribution diagram of the metasurface with parameters α ≠ 0 and parameter β ≠ 0; Figure 4 (d) in is the simulated Q factor of TE2 as a function of parameter α, fitted by the inverse square law; Figure 4 (e) in is a graph of the variation of each chiral source with parameter α with parameter β fixed; Figure 4 (f) in is a graph of the CD and Q factor obtained from the spectrum as a function of parameter α with parameter β fixed.

[0042] Figure 5 This is the spectrogram of the device for generating high-efficiency intrinsic circular dichroism of the present invention varying with parameter α at a fixed parameter β.

[0043] Figure 6 Schematic diagram of the intrinsic CD spectra calculated by the finite element method (FEM) and the proposed coupled mode theory (CMT) under different parameters α and β, where the solid line represents the calculation result of CMT, and the solid circles represent the calculation results of FEM. Specific implementation manners

[0044] To make the objectives, technical solutions and advantages of the present invention clearer, the following will further describe the implementation manners of the present invention in detail with reference to the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0045] Embodiment 1:

[0046] This embodiment provides a device for generating high-efficiency intrinsic circular dichroism, including a substrate and a metasurface attached to the substrate. The metasurface is composed of periodically arranged nanostructures. The geometric shape of the nanostructures is a combination of double rectangles, and the combination of double rectangles includes two interconnected small rectangles and a large rectangle, and the area of the large rectangle is larger than that of the small rectangle; the geometric shape and arrangement of the nanostructures are designed to generate different optical responses to left-handed circularly polarized light (LCP) and right-handed circularly polarized light (RCP), so as to achieve high-efficiency intrinsic circular dichroism.

[0047] The nanostructures have in-plane mirror symmetry breaking, and partial etching of the nanostructures is used to break the out-of-plane mirror symmetry of the nanostructures to obtain intrinsic circular dichroism; among them, under the condition of non-breaking symmetry, the side length of the air hole of the nanostructure is l, the height of the nanostructure is H, and the out-of-plane mirror asymmetry parameter is defined as β = h / 2H, where h represents the etching depth; breaking the in-plane mirror symmetry of the nanostructures becomes a combination of double rectangles composed of a large rectangle and a small rectangle with unequal areas after changing the length of δ, and the in-plane mirror asymmetry parameter is defined as α = δ / l, where δ represents the width of the small rectangle in the combination of double rectangles.

[0048] Among them, the in-plane mirror asymmetry parameter α of the nanostructures is 0.177, and the out-of-plane mirror asymmetry parameter β of the nanostructures is 0.155.

[0049] On this basis, BIC is introduced to enhance circular dichroism. BIC is a bound state that can achieve perfect localization of the electromagnetic field even when the eigenstate of the system is embedded in the continuous spectrum of light radiation. Theoretically, BIC is an idealized model, but by breaking the symmetry of the structure, BIC can be transformed into quasi-BIC with a high Q factor, which can effectively enhance the interaction between light and matter, and then significantly enhance the circular dichroism signal.

[0050] Furthermore, the substrate is made of a low-refractive-index material, silica (SiO2), to ensure low loss and high transmittance when light passes through.

[0051] Furthermore, the nanostructure of the metasurface is made of silicon (Si) material, with a height of 500 nm, a width of 480 nm, and an etching depth of 345 nm.

[0052] Furthermore, the geometric shape of the nanostructure is a combination of two rectangles with unequal areas, and its arrangement is a periodic array with a period of 905 nm.

[0053] Furthermore, the design and optimization of the metasurface are as follows:

[0054] Design goal: Achieve a high-efficiency intrinsic circular dichroism response in the near-infrared band from 1400 nm to 1700 nm.

[0055] Simulation optimization: Use the finite element method (FEM) to optimize the geometric parameters of the nanostructure to ensure that it supports the symmetric-protected BIC mode and enhances the circular dichroism response.

[0056] Optimization result: Determine the optimal geometric parameters of the nanostructure through simulation to achieve an intrinsic circular dichroism efficiency greater than 0.9 within the target wavelength range.

[0057] Example 2:

[0058] BIC-based resonant metasurfaces can be used to enhance circular dichroism. BIC is a bound state that can achieve perfect localization of the electromagnetic field when the system's eigenstate is embedded in the continuous spectrum of light radiation. In theory, BIC is an idealized model, but by breaking the symmetry of the structure, BIC can be converted into a quasi-BIC with a high Q factor, which can effectively enhance the interaction between light and matter, thereby achieving a significant enhancement of the circular dichroism signal. Previous methods for enhancing intrinsic circular dichroism using quasi-BIC require tilting or stretching the microstructure in order to simultaneously break the in-plane and out-of-plane mirror symmetry of the structure, which increases the difficulty of microstructure processing and preparation. Based on this, the present embodiment provides a device for generating high-efficiency intrinsic circular dichroism, by constructing a metasurface with broken in-plane symmetry, and at the same time partially etching the nanostructure (rather than tilting or stretching the microstructure in the past) to break the out-of-plane mirror symmetry of the structure, thereby constructing a chiral quasi-BIC metasurface, thereby generating high-efficiency intrinsic circular dichroism.

[0059] Figure 1 (a) shows the schematic diagram of the structure of the device. The metasurface is attached to the SiO2 substrate and consists of periodically arranged Si nanostructures. Under normal incidence, its reflection response to LCP and RCP shows a huge difference, namely circular dichroism. Figure 1 (b) shows the cross-sectional and top-down views of the device, where the period P = 905 nm, the side length of the air hole l = 480 nm without breaking symmetry, the Si height H = 500 nm, and the out-of-plane asymmetry parameter β = h / 2H, where h represents the etching depth. Breaking the in-plane symmetry (i.e., changing the length δ) results in a geometric configuration of two rectangles of unequal areas, where the in-plane asymmetry parameter α = δ / l. Figure 1 (c) shows the band structure diagram of the device under the conditions of α = 0, β = 0, that is, completely symmetrical. The horizontal axis represents the wave vector and the vertical axis represents the normalized frequency. In this frequency domain, both modes have infinite quality factors at the Γ point, as shown in the figure. Figure 1 (d) in the figure, we prove that it supports the symmetric protected BIC mode. Without loss of generality, we first consider TE2.

[0060] Example 3:

[0061] This embodiment provides a method for preparing a device that generates high-efficiency intrinsic circular dichroism. The flow chart of the preparation method is as follows: Figure 2 As shown, the nanostructure of the metasurface is prepared by nanofabrication technology, including electron beam lithography, focused ion beam etching or nanoimprint technology; the preparation method includes:

[0062] Step 1: Coat the substrate with photoresist and transfer the nanostructure pattern onto the photoresist using electron beam lithography.

[0063] Step 2: Transfer the pattern on the photoresist mask to the substrate or functional material layer by dry etching (such as reactive ion etching) or wet etching technology to form nanostructures with specific geometries and dimensions;

[0064] Step 3: Remove the residual photoresist, and clean and surface-treat the metasurface (such as passivation or functionalization) to improve its optical performance and environmental stability.

[0065] Further, Step 2 specifically includes:

[0066] Dry etching: Transfer the pattern on the photoresist mask to the substrate or functional material layer by using reactive ion etching (RIE) or inductively coupled plasma etching (ICP) technology; control the depth and sidewall morphology of the nanostructures by adjusting the flow rate, power, and etching time of the etching gas (such as sulfur hexafluoride, carbon tetrafluoride, oxygen, etc.) to ensure that its geometry and dimensions meet the design requirements.

[0067] Wet etching: Selectively etch the substrate or functional material layer with a chemical etching solution (such as potassium hydroxide solution or hydrofluoric acid solution), and achieve high-precision shaping of the nanostructures by controlling the concentration, temperature, and etching time of the etching solution.

[0068] Further, Step 3 specifically includes:

[0069] Removing the residual photoresist: Thoroughly remove the photoresist residue using an organic solvent (such as acetone or N-methylpyrrolidone) or oxygen plasma cleaning technology to ensure a clean surface of the nanostructures.

[0070] Surface passivation treatment: Deposit a passivation film (such as aluminum oxide or silicon dioxide) on the surface of the nanostructures by atomic layer deposition (ALD) or chemical vapor deposition (CVD) technology to improve its environmental stability and corrosion resistance.

[0071] Further, the preparation method further includes the following quality control steps:

[0072] Morphology characterization: Characterize the morphology of the nanostructures using a scanning electron microscope (SEM) or an atomic force microscope (AFM) to ensure that its geometric parameters meet the design requirements.

[0073] Optical performance testing: Test the optical performance of the metasurface using a circular dichroism spectrometer or an ellipsometer to verify the intensity and band range of its chiral optical response.

[0074] Process optimization and iteration: Optimize and adjust the preparation process according to the test results until the desired intrinsic circular dichroism efficiency (exceeding 0.9) and optical performance are achieved.

[0075] Specifically, taking the following preparation method as an example, first, a layer of ZEP520 resist layer is spin-coated on a clean SOI wafer. Then, an electron beam lithography (EBL) tool is used to define a pattern on the resist layer, and the pattern is fixed through a developing process. Next, an inductive lithography tool is used to transfer the resist layer pattern to the top silicon layer. Finally, an inductively coupled plasma (ICP) etching technique is used to transfer the resist pattern to the top silicon layer, and a nano-etching technique is used to remove the remaining resist. An NMP solution is used to remove the remaining resist.

[0076] Example 4:

[0077] Figure 3 The evolution of the eigenpolarization map of TE2 in momentum space with respect to the asymmetry parameters α and β is given. For the case where the structure has both in-plane and out-of-plane mirror symmetries (α = 0, β = 0), the BIC is located at the Γ point in momentum space, which is also the V point, as shown in (a) of Figure 3 As shown in (b) of Figure 3 once a non-zero α is introduced, the V point with integer charge is decomposed into a pair of half-charge chiral C points that are centrosymmetric about the Γ point, where the solid black dot and the solid gray dot have right-handed circular polarization and left-handed circular polarization, respectively. On this basis, if a non-zero β is introduced, the chiral C points will gradually approach the Γ point. For an appropriate combination of α and β, such as α = 0.177 and β = 0.155, the C points will further move to the Γ point, thus achieving an eigen-chiral BIC, as shown in (c) of Figure 3

[0078] Example 5:

[0079] To evaluate the influence in generating chiral radiation, the helicity density (HD) is introduced, which is a concept in electromagnetics or photonics that describes the local helicity storage of the electromagnetic field and includes two types. One is the reactive helicity density (RHD), defined as: χ r = c / ωRe(B*D), which describes the helicity propagated in the electromagnetic field, such as the helicity carried by a radiation field (such as far-field circularly polarized light); the other is the optical chirality density (OCD), also known as the active helicity density, defined as: χ i = c / ωIm(B*D), which describes the locally stored, non-radiative helicity (such as the helicity of a near-field or bound-state field). Here, c represents the speed of light in vacuum, ω represents the frequency of the incident light, B* represents the complex conjugate of the magnetic induction intensity, and D represents the electric displacement vector.

[0080] ​Although OCD has been specifically used to evaluate the chiral asymmetry of electromagnetic fields, due to the significant influence of RHD in generating chiral radiation, this chiral radiation dominates in optical chirality. In fact, OCD has long been regarded as the standard for measuring the local chirality of electromagnetic fields. However, it only applies to the simplest case of considering homogeneous, non-magnetic, and lossless media without nanostructures. For the general case involving multiple material regions with boundaries, the local chirality of the electromagnetic field is jointly determined by OCD and RHD, both of which are sources of chiral flux. In addition, in a high-Q resonant system, the contribution of RHD often outweighs that of OCD.

[0081] Figure 4 (a) in Figure 4 (b) in Figure 4 (c) in shows the evolution of the RHD distribution of TE2 with the asymmetry parameters α and β. For the case where the structure has both in-plane and out-of-plane mirror symmetries, i.e., α = 0, β = 0, due to the C4 symmetry of the structure, the Γ point in the Brillouin zone supports a symmetry-protected BIC, as shown in Figure 4 (a). For the lossless BIC state, due to time-reversal symmetry, the RHD retrieved on the structure is 0. After introducing the in-plane asymmetry parameter α, the BIC transforms into a quasi-BIC, and the variation of its Q factor follows an inverse-square law with respect to the parameter α: Q ∼ α -2 , as shown in Figure 4 (d). The structural asymmetry directly leads to the chirality of the electromagnetic near field, thereby generating a non-zero RHD on the structure. Since RHD is a parity scalar, it exhibits an antisymmetric distribution with respect to the central x-y plane, and its integral over the entire space will still remain zero. On this basis, to break the out-of-plane balance of RHD, the out-of-plane asymmetry parameter β is introduced, resulting in the generation of chiral radiation, as shown in Figure 4 (c). Here the parameter β is fixed at 0.155. To select the optimal α for achieving the maximum chirality, the RHD integrated over the entire space with OCD is calculated as a function of α, and it is observed that the RHD first increases rapidly with α and reaches a maximum at α = 0.177. In addition, it is also observed that the contribution of RHD is much greater than that of OCD, which means that RHD plays a dominant role in generating chiral radiation, as shown in Figure 4 (e). According to this theory, CD also increases with the increase of α and reaches a maximum of 0.96 at α = 0.177, where CD is defined as CD = R LCP - R RCP , which is the difference between the LCP and RCP reflectivities for spectral calculation. Here Q is calculated by Q = ω0 / (2γ), where the resonance frequency ω0 and the radiation loss γ are retrieved from the simulated spectrum. There is still a high-quality factor of over 2600 when CD reaches its peak.

[0082] Example 6:

[0083] This embodiment provides a method for generating high-efficiency intrinsic circular dichroism. By using the above device, fixing β=0.155, and performing numerical simulation with different in-plane asymmetry parameters α, high-efficiency intrinsic circular dichroism is obtained at different resonant wavelengths. Figure 5 (a) and Figure 5 (b) represents R under RCP incident RR With R LR Component spectrum, Figure 5 (d) and Figure 5 (e) in the figure represent R under LCP incident LL With R RL Component spectrum. Among them R ij The subscripts (i=R, L, j=R, L) represent RCP and LCP, respectively, where j represents the incident light and i represents the outgoing light. Figure 5 (c) and Figure 5 (f) in the figure represents the CD response at two different resonance positions. Here CD is defined as CD = R LL +R RL -R RR -R LR It can be seen that the circular dichroism mainly comes from R LL With R RR It can be seen that when α = 0.196 and 0.177, the highest efficiency of intrinsic circular dichroism is obtained at 1516.6nm and 1526.6nm respectively, among which the intrinsic circular dichroism efficiency at 1526.6nm is extremely high, exceeding 0.9.

[0084] Embodiment seven:

[0085] This embodiment provides a simplified analytical model structure for evaluating chiral quasi-continuum bound-state circular dichroism in a device for generating high-efficiency intrinsic circular dichroism. This model structure can link radiative loss with circular dichroism by introducing an asymmetric parameter and accurately estimate the corresponding circular dichroism spectrum, facilitating a simplified analysis of the circular dichroism of chiral quasi-BIC metasurfaces. Using coupled mode theory (CMT), based on time reversal symmetry and energy conservation, this model structure can express the expression for the reflected / transmitted component as follows:

[0086]

[0087] Among them, A j 、B j 、C j 、D jdenotes the fitting parameter, γ is the attenuation rate caused by radiation, ω0 is the resonance frequency, and ω is the frequency of the incident light;

[0088] Define CD = R LL +R RL -R RR -R LR For the intrinsic CD, due to the cross-polarization components R RL = R LR Therefore, it can be written as:

[0089]

[0090] where the subscripts of R ij (i = R, L, j = R, L) represent right-handed circularly polarized light and left-handed circularly polarized light respectively, j represents the incident light, i represents the outgoing light, m and n are the resonance coupling coefficients of the structure with incident left-handed circularly polarized light and right-handed circularly polarized light respectively, and r' represents the background scattering coefficient.

[0091] The intrinsic CD can be simplifiedly analyzed and calculated using Equation (2). Figure 6 The intrinsic CD spectra calculated by FEM and the proposed CMT for different non-symmetric parameters α and β are shown. The solid line is the calculation result of CMT, and the solid circles represent the calculation results of FEM. It can be seen from the figure that for different combinations of α and β, the calculation results obtained by CMT are in good agreement with those of FEM, indicating that the proposed CMT can accurately estimate the intrinsic CD spectra.

[0092] In summary, the present invention provides a device for generating high-efficiency intrinsic circular dichroism and its preparation method. By constructing a nanostructure with in-plane mirror symmetry breaking and simultaneously partially etching the nanostructure to break the out-of-plane mirror symmetry of the structure, high-efficiency intrinsic circular dichroism is obtained. The device achieves an intrinsic circular dichroism efficiency greater than 0.9 in the near-infrared band from 1400 nm to 1700 nm, has broadband working ability and strong field localization effect. Using mature nanofabrication technology simplifies the preparation process, reduces costs, and improves environmental stability through material selection and surface treatment. The present invention can be widely applied in the fields of chiral molecule detection, circularly polarized light detectors, chiral displays, biomedical imaging, etc., providing an efficient, stable and easily integrated solution for the development of chiral optical technologies.

[0093] In this text, specific embodiments are used to illustrate the principles and implementation manners of the present invention. The description of the above embodiments is only for helping to understand the method of the present invention and its core idea. It should be noted that for those of ordinary skill in the art in this technical field, without departing from the principles of the present invention, several improvements and modifications can still be made to the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention. Therefore, the protection scope of this invention patent shall be subject to the appended claims.

Claims

1. An apparatus for generating high-efficiency intrinsic circular dichroism, characterized in that, It includes a substrate and a metasurface attached to the substrate. The metasurface is composed of periodically arranged nanostructures. The geometric shape of the nanostructures is a combination of two rectangles, and the combination of two rectangles includes two small rectangles and a large rectangle that are interconnected. The area of the large rectangle is larger than that of the small rectangle. The geometric shape and arrangement of the nanostructures are configured to produce different optical responses to left-handed circularly polarized light and right-handed circularly polarized light; the nanostructures have broken in-plane mirror symmetry, and partial etching of the nanostructures is used to break the out-of-plane mirror symmetry of the nanostructures to obtain intrinsic circular dichroism; among them, under the condition of unbroken symmetry, the side length of the air hole of the nanostructure is l, the height of the nanostructure is H, and the out-of-plane mirror asymmetry parameter is defined as β = h / 2H, where h represents the etching depth; breaking the in-plane mirror symmetry of the nanostructure becomes a combination of two rectangles with unequal areas of a large rectangle and a small rectangle after changing the length of δ, and the in-plane mirror asymmetry parameter is defined as α = δ / l, where δ represents the width of the small rectangle in the combination of two rectangles.

2. The device for generating high-efficiency intrinsic circular dichroism according to claim 1, characterized in that, The substrate is low-refractive-index material silicon dioxide.

3. The device for generating high-efficiency intrinsic circular dichroism according to claim 2, characterized in that, The nanostructures of the metasurface are made of silicon material, with a height of 500 nm, a width of 480 nm, and an etching depth of 345 nm.

4. A device for generating highly efficient intrinsic circular dichroism according to claim 3, characterized in that, The arrangement period of the nanostructures is 905 nm.

5. An apparatus for generating high-efficiency intrinsic circular dichroism according to claim 4, characterized in that, The metasurface is configured to produce circular dichroism in the near-infrared band, and its working wavelength range is from 1400 nm to 1700 nm.

6. The device for generating high-efficiency intrinsic circular dichroism according to claim 1, characterized in that, The in-plane mirror asymmetry parameter α of the nanostructures is 0.177, and the out-of-plane mirror asymmetry parameter β of the nanostructures is 0.

155.

7. A preparation method of an apparatus for generating high-efficiency intrinsic circular dichroism according to any one of claims 1-6, characterized in that, The preparation method includes: Step 1: Coat a photoresist on the surface of the substrate, and transfer the pattern of the nanostructures to the photoresist through electron beam lithography technology; Step 2: Adopt dry etching or wet etching technology to transfer the pattern on the photoresist mask to the substrate or the functional material layer to form nanostructures with the above geometric shapes and sizes; Step 3: Remove the residual photoresist, and clean and surface-treat the metasurface to improve its optical performance and environmental stability.

8. The preparation method according to claim 7, wherein Step 2 specifically includes: Dry etching: Adopt reactive ion etching or inductively coupled plasma etching technology to transfer the pattern on the photoresist mask to the substrate or the functional material layer; by adjusting the flow rate, power, and etching time of the etching gas, control the depth and sidewall morphology of the nanostructures to ensure that their geometric shapes and sizes meet the design requirements; Wet etching: Use a chemical etching solution to selectively etch the substrate or the functional material layer, and achieve high-precision shaping of the nanostructures by controlling the concentration, temperature, and etching time of the etching solution; Step 3 specifically includes: Removing the residual photoresist: Use an organic solvent or oxygen plasma cleaning technology to thoroughly remove the photoresist residue and ensure the cleanliness of the nanostructure surface; Surface passivation treatment: Deposit a passivation film on the surface of the nanostructures through atomic layer deposition or chemical vapor deposition technology to improve its environmental stability and corrosion resistance.

9. The preparation method according to claim 8, wherein, The preparation method further includes the following quality control steps: Morphological characterization: Use scanning electron microscopy or atomic force microscopy to characterize the morphology of the nanostructure to ensure that its geometric parameters meet the design requirements; Optical property testing: Test the optical properties of the metasurface through a circular dichroism spectrometer or ellipsometer to verify the intensity and wavelength range of its chiral optical response; Process optimization and iteration: Optimize and adjust the fabrication process according to the test results until the expected intrinsic circular dichroism efficiency and optical properties are achieved.

10. A simplified analytical model structure for evaluating the chiral quasi-continuous spectral bound state circular dichroism in a device for generating high-efficiency intrinsic circular dichroism according to any one of claims 1-6, characterized in that, Based on the coupled mode theory, this model structure, based on time-reversal symmetry and energy conservation, writes the expression of the reflection component / transmission component in the following form: Among them, A j , B j , C j , D j represent fitting parameters, γ is the attenuation rate caused by radiation, ω0 is the resonance frequency, and ω is the frequency of the incident light; Define CD = R LL +R RL -R RR -R LR , for the intrinsic CD, due to the cross-polarization component R RL = R LR , so it is written as: where R ij (i = R, L, j = R, L) The subscripts respectively represent right-handed circularly polarized light and left-handed circularly polarized light. j represents the incident light, i represents the outgoing light, m and n are the resonance coupling coefficients of the structure with incident left-handed circularly polarized light and right-handed circularly polarized light respectively, and r' represents the background scattering coefficient.

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